Fungi of the genus Coelomomyces are worldwide in distribution and cause high mortalities in mosquito populations. They are not likely to cause environmental pollution since they are host specific for mosquitoes and do not grow saprobically. Nevertheless, the potential of these fungi has not been realized because no one has successfully cultured them in vitro or precisely determined the conditions under which they attack mosquito larvae most effectively. There is ample evidence, however, that they can be established in new localities where they persist and cause disease from year to year. Several field collected species will be examined for their adaptability to laboratory study; but C. psorophorae, a species with wide host range (4 mosquito genera) and distribution (much of North America) will be emphasized. We propose to determine with the one species the chemical factors which enhance sporangial dehiscence and the ecological parameters necessary for disease induction, to describe the details of infection and disease development, and to culture the fungus in vitro.